1. Field of the Invention
The present invention relates to a so-called side-on type photomultiplier on which light to be measured is incident from the side surface of its container and, more particularly, to make uniform the output waveform and improve the signal-to-noise S/N ratio of a photomultiplier.
2. Related Background Art
FIGS. 1 and 2 show a conventional photomultiplier. This photomultiplier is generally called a side-on type photomultiplier, and light as the measurement target is incident on the photomultiplier from the side surface of its glass bulb 1, which is a transparent sealed container. Light is transmitted through the glass bulb 1 and is incident on the photoelectric surface of a reflection type photocathode 2. As a result photoelectrons are emitted from the photoelectric surface and sent to an electronic multiplier section 3 constituted by a plurality of stages of dynodes 3a to 3d. The photoelectrons are sequentially multiplied by the electronic multiplier section 3, and the multiplied photoelectrons are collected as the output signal by an anode 4.
In order to guide the photoelectrons emitted from the photocathode 2 to the first-stage dynode 3a, a grid electrode 6 is arranged between a light-incident portion 5 of the glass bulb 1 and the photocathode 2 and set to the same potential as that of the photocathode 2. Various types of grid electrodes 6 are available. For example, a thin conductor wire is arranged literally in a grid-like manner (not shown) to constitute a grid electrode 6, or as shown in FIG. 1, one thin conductor wire 6c is spirally wound on two support rods 6a and 6b to constitute a grid electrode 6.
In the conventional photomultiplier as described above, because the grid electrode 6 is arranged in front of the photocathode 2, light incident on the photocathode 2 through the glass bulb 1 is partly scattered and absorbed by the conductor wire 6c of the grid electrode 6. Even if the incident light is uniform, a part of the light does not reach the photocathode 2. In general, the grid electrode 6 has a transmittance of 75%. Hence, 25% of the light does not reach the photocathode 2.
FIG. 3 is a graph showing the relationship between the position of a light spot formed and the output (relative value) of the anode 4 serving as the collector electrode when spot light is radiated as it is moved from an upper point a to a lower point b along the plane 2--2 of FIG. 1. Referring to FIG. 3, the output 60 is not uniform. The position of a recess in the output corresponds to the position of the conductor wire 6c of the grid electrode 6. It is apparent that the transmittance is decreased at this position.
As countermeasures against the problem of the decrease in transmittance, means disclosed in Japanese Patent Laid-Open Nos. 53-18864 and 55-29989 are known.
As shown in FIG. 4, according to the means disclosed in Japanese Patent Laid-Open No. 53-18864, a glass plate 7 having a transparent conductor film formed on its surface is used in place of the grid electrode 7.
When light is transmitted through a glass material, however, a loss occurs due to absorption or scattering. When the glass plate 7 is arranged in a glass bulb 1, light is transmitted through the glass material twice, doubling the loss.
Another problem arises in manufacture. More specifically, in the conventional manufacturing process of a photocathode 2, an alkali metal for forming the photoelectric surface flows as indicated by broken lines in FIG. 4 to reach the photoelectric surface. When the glass plate 7 is arranged in the moving path of the alkali metal, the alkali metal cannot be uniformly guided, making it very difficult to form a uniform photoelectric surface.
As shown in FIG. 5, according to the means disclosed in Japanese Patent Laid-Open No. 55-29989, although a grid electrode 6 is used, the grid density constituted by a conductor wire 6c of the grid electrode 6 is set high in a portion 6d close to a portion of the grid electrode 6 which is coupled to a photocathode 2 and low in a portion 6e through which most of the incident light is transmitted.
When the grid density of the grid electrode 6 is set low only partly, although the transmittance is increased as compared to that obtained in the conventional arrangement shown in FIG. 1. But the conductor wire 6c of the grid electrode 6 still serves as an obstacle to decrease the transmittance, leaving the problem unsolved. Different transmittances in different portions of the grid electrode 6 mean different transmittances of light to be incident in different portions on the photocathode 2. This causes non-uniformity in the sensitivity of the photocathode 2.